Process for controlling a twin clutch

ABSTRACT

The invention relates to a process for controlling a twin clutch transmission with two partial drive trains with respectively a friction clutch interposed between an internal combustion engine and the partial drive train. If the transmission capacity of a friction clutch falls below the engine torque, engine intervention takes place. If the clutch temperature rises further above a default value, an emergency operation is initiated, in which the affected partial drive train is deactivated by opening the affected friction clutch, a preselection strategy used in the other partial drive train is changed and the other partial drive train is activated.

This claims the benefit of German patent Application No 10 2008 017558.7 filed Apr. 7, 2008 and hereby incorporated by reference herein.

The present invention relates to a process for controlling a twin clutchtransmission with two partial drive trains that can be coupledrespectively with an internal combustion engine by means of a frictionclutch.

BACKGROUND

Twin clutch transmissions and processes for their control are known. Ina twin clutch transmission, both partial drive trains are operatedalternately by closing the respective associated friction clutch. Duringthe operation of one partial drive train, the other one is prepared forthe next operation. This means, among others, that according to apreselection strategy, the next likely gear in the driving operation isalready preselected; for instance, in a twin clutch transmission withgearwheel pairs comprising a fixed gear and a selectable mating gear isalready engaged, wherein the shifting clutch connecting the mating gearwith the corresponding shifter shaft is already shifted or prepared forshifting. Once the gear to be activated is engaged, shifting takes placeby overlapping the two friction clutches, thus by concurrent actuationof the friction clutch, from the closed into the opened state in theactive partial drive train, and from open to the closed state of thefriction clutch in the partial drive train to be activated anew. In thethen deactivated partial drive train, a further gear is subsequentlyprepared for shifting or engaged immediately according to thepreselection strategy.

Due to stress on friction clutches during start-up processes, inparticular under heavy load, successive rapid gearshifts, thusoverlapping gearshifts being constantly associated with clutch slip canimpair the transmission capacity of the friction clutches; so that,under certain circumstances, the transmission capacity drops, thus themaximum clutch torque is no longer transmitted because, for instance,the friction linings of the friction clutches fade due to decreasedcoefficient of friction. Subsequently, the prevailing torque of theinternal combustion engine in that case can no longer be transmitted andundesired slip occurs. To rectify this, the engine torque is reduced.This can jeopardize the propulsion of the vehicle equipped with the twinclutch transmission.

SUMMARY OF THE INVENTION

An object of the present invention is therefore to control a twin clutchtransmission such that the risk of the vehicle coming to a standstillcan be avoided or delayed at least. Furthermore, the affected frictionclutch should be protected from damages.

The present invention provides a process for controlling a twin clutchtransmission with two partial drive trains that respectively featuremultiple selectable gears, and of which one partial drive train with anactively engaged gear is respectively operated, whilst in the otherpartial drive train, according to a preselection strategy, a gear forsubsequent driving operation is engaged. In this case, a friction clutchis assigned respectively to one partial drive train that is activated bya control device via a travel- or force-controlled actuator and thattransmits maximum clutch torque upon attaining or exceeding a prescribedthreshold value. In case this maximally transmittable clutch torque isnot sufficient to transmit the torque developed by an internalcombustion engine driving the twin clutch transmission, reduction of thedriving engine torque will be required. During this reduction of enginetorque, the affected friction clutch can be closed maximally in order toenable a probably necessary adjustment of the friction clutch viaadditional travel. In case, by falling short of the maximum clutchtorque of a friction clutch, a clutch temperature value associated withthis friction clutch is exceeded, through which fading of the frictionclutch can be detected, an emergency operation will be initiated. Thisemergency comprises at least deactivation of the clutch assigned to thispartial drive train, by opening the friction clutch, by changing thepreselection strategy and by activating the other partial drive train.

At least one of the friction clutches is a self-opening -closed frictionclutch. This means that the friction clutch is pressed-closed by anactuator, for instance a lever actuator, against the effect of anopening force, so that, upon deactivation of the actuator, the frictionclutch independently opens and hence prevents interlocking thetransmission, when, for instance, both friction clutches transmit torqueto the partial drive trains whilst gears are engaged in both partialdrive trains.

Upon recovery of the friction clutch or rather its friction linings, itcan be provided that, depending upon falling short of the clutchtemperature, the emergency operation is deactivated again and return ismade to normal operation of the twin clutch transmission, in which,again, both partial drive trains and the preselection strategy intendedfor this purpose are used. Through the selection of clutch temperature,deactivation of the emergency operation can take place directly, since,in general, the properties of the friction lining and its degree ofdamage are a direct function of the temperature. A temperature sensor inthe area of the friction clutches can be provided as clutch temperaturefor each respective friction clutch. It has proved, however,advantageous to use a clutch temperature model to determine the clutchtemperature, wherein, for instance, clutch temperature can be derivedfrom the torque and slip transmitted via the individual friction clutch.In this way, corresponding temperature sensors can be omitted to savecosts.

In addition, it has proved advantageous if a damage index is calculatedbesides the clutch temperature. A damage index indicates the damage tothe friction clutch as well as their components dependent upon thefrictional power put into the clutch. Depending on such a damage index,for instance, protective measures can be initiated for the frictionclutches. In particular, a damage index can be used to monitor theclutch pressure plates made of cast iron. In this regard, empiricaland/or temperature variations determined via computation programsdependent upon torque input, slip, temperature and/or temperaturegradient can be correlated to the damage behavior so that in good timeprior to the damage expected, appropriate measures like the activationof emergency operation can be met. If, thereby, a pressure plate of anindividual friction clutch is affected, this friction clutch can beopened at least temporarily, until it has cooled down below a valueuncritical to the pressure plate. If the common pressure plate or ratherboth friction clutches are affected, a warning message can be output, ingood time, with subsequent lay up of the vehicle at short notice.

The damage index is formed advantageously as a routine for monitoringthermal shock of a friction clutch pressure plate under frictionalengagement. Upon detection of thermal shock, the affected frictionclutch can be opened for a predetermined time interval, wherein thelength of the time interval can be made to depend upon the empirical orcalculated cooling behavior of the pressure plate. It is understandablethat, for this purpose, corresponding safety factors can be providedwith preset or adjustable extra time. By monitoring possible occurrenceof thermal shock, if necessary, a high-quality and hence expensive castmaterial, which withstands thermal shock without monitoring, can bedispensed with; in that, in individual cases, by monitoring thermalshock at the right time for the driver, with only a little scope of lossof comfort, an emergency operation can react to thermal shock, upondetection.

In addition, a routine can be provided additionally, in which aself-opening function for an application with at least one self-openingfriction clutch is checked within a preset time interval, and in casethe self-opening function is absent, the emergency operation will beinitiated. If the self-opening function of a friction clutch is faulty,it will no longer open or it will no longer open correctly so thatduring overlapping gearshift, particularly the twin clutch transmissioncan be interlocked when gears are engaged in both partial drive trains.If the routine for monitoring the self-opening function results in adefault number of unsuccessful self-openings, for instance a numberbetween one and five, then the emergency operation will be activated.The self-opening function can be temporarily faulty or it may fail, forinstance at very high or very low temperatures, so that the vehicle mustnot be advantageously laid up immediately. On the contrary, a regularverification routine can be carried out in a further procedure. Ifself-opening of the friction clutch or clutches occurs again withouterror after a default number of verifications, the emergency operationcan be switched off again and the normal operation initiated. What iscertainly advantageous in all cases is the documentation in an errormemory of these incidents so that, in case of multiple occurrences oftemporary faults, a warning message is output so that if need be, aworkshop can be visited. In addition, a warning to the driver is outputafter a default number of failed self-openings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in detail based on FIGS. 1 and 2, as follows:

FIG. 1 shows an exemplary embodiment of a routine for monitoring thetorque transmission behavior of a friction clutch and

FIG. 2 shows an exemplary embodiment of a routine for application of adamage index to a friction clutch.

DETAILED DESCRIPTION

FIG. 1 shows an exemplary embodiment of a routine 1 for monitoring andcontrolling a twin clutch transmission, depending upon the state of thefriction clutches. In doing so, the routine 1 for both friction clutchesof the twin clutch transmission can be performed in series, thussuccessively or in parallel by means of indexing the individualquantities for assignment of individual values to the correspondingfriction clutch. The routine 1 is started in block 2. In the subsequentbranch 3, the maximum clutch torque transmittable via the frictionclutch TrqClCapMax is determined. For this purpose, quantities like theprevailing engine torque TrqEngMax and the friction clutch slip or amodel of the friction clutch, if necessary, as adapted in the drivingoperation can be used. If the maximum clutch torque TrqClCapMax isgreater than the prevailing engine torque TrqEngMax, then normaloperation is continued in block 8. Under normal operation, oneunderstands an overlap of the two friction clutches when driving with aclosed friction clutch of the active partial drive train andpreselection of a gear whilst the friction clutch is open, according toa default preselection strategy in the other partial drive train, and agearshift desire, if a control device is present.

In branch 3, it is verified whether, if applicable, a smaller maximumclutch torque TrqClCapMax falls short of a default fraction of enginetorque TrqEngMax by a factor x1. If this is the case, then it is decidedin branch 4, whether a fading temperature T_(Fading) is attained orexceeded, based on a clutch temperature model that determines andintegrates the thermal capacities of clutch component parts, the torqueinput into the friction clutch and the slip that occurs in the process.If this is the case, then an emergency operation is initiated in block5, which shuts down the affected partial transmission, in that thefriction clutch is opened and/or the engaged gear is disengaged undercorresponding conditions. The vehicle can then continue driving with theother partial drive train, by changing the preselection strategy. Ifclutch temperature TcCL is lower than the fading temperature T_(Fading)then a defective friction clutch may be evident. In this case, thepartial transmission is shut down in block 6 and an engaged gearpreferably disengaged. A warning message to look for a workshop isoutput advantageously.

If the maximum clutch torque TrqClCapMax lies between the priordescribed states, a torque request to limit the maximum engine torque tobe developed TrqEngMax will be output, for instance, via CAN-bus or viaa separate line to the control device of the internal combustion engine,in order to minimize the load on the friction clutch.

The routine 1 is preferably passed through cyclically, if necessary, inorder to adapt the control system to the changing quantities.

FIG. 2 describes a process for evaluating damage on one or both frictionclutches based on the damage index IdxDamage, by means of routine 9.This is started in block 10. In the following branch 1, the damage isdivided, based on the damage index IdxDamage with three index quantitiesS1, S2, and S3, in four damage classes, against which, in the blocks 12,13, 14, and 15, corresponding measures are taken. If the damage is equalto zero or is negligible, then normal operation will be sustained inblock 12. In the second damage class, a light clutch protection occursin block 13. This includes a changed preselection strategy, in which theother drive train is preferred, in that, for instance, the gears engagedthere are run at higher revs or gearshifts occur with traction forceinterruptions on the same partial drive train. Engine intervention canoccur selectively in this class. In the next class, clutch protectioncan be made further intensive in block 14, in that, for instance, themeasures of block 13 are made more intensive or one partial drive trainis temporarily deactivated. In the last damage class in block 15, athermal shock can be assumed, in which a high temperature gradientendangers cast iron parts, for instance the pressure plate of theaffected friction clutch. This can for instance burst under overload. Tocircumvent this danger, the drive train is temporarily deactivated forso long until the damage index IdxDamage has again fallen below theindex quantity S3. The index quantities S1, S2, and S3 can bedetermined, for instance, based on material and design data of theclutches.

LIST OF REFERENCE SYMBOLS

-   -   1 routine    -   2 block    -   3 branch    -   4 branch    -   5 block    -   6 block    -   7 block    -   8 block    -   9 routine    -   10 block    -   11 branch    -   12 block    -   13 block    -   14 block    -   15 block    -   IdxDamage damage index    -   S1 index quantity    -   S2 index quantity    -   S3 index quantity    -   TcCl clutch temperature    -   TFading fading temperature    -   TrqClCapMax maximum clutch torque    -   TrqEngMax engine torque    -   x1 proportional factor

1. A method for controlling a twin clutch transmission with two partialdrive trains, which respectively feature multiple executable gearshifts,and in which one partial drive train is operated respectively with onegear actively engaged and in which, in another partial drive trainaccording to a preselection strategy, a gear is engaged for a subsequentdriving operation, in which a friction clutch is assigned respectivelyto one partial drive train, which is actuated by an actuator activatedtravel- or force-dependently by a control device and, upon attaining orexceeding a default threshold, transmits maximum clutch torque, themethod comprising: if the maximum clutch torque of a friction clutchfalls short of a predefined threshold value and once the clutchtemperature assigned to this friction clutch is exceeded, an emergencyoperation is initiated, in which the partial drive train correspondingto the affected clutch is deactivated by opening the friction clutch,changing the preselection strategy, and activating the other partialdrive train.
 2. The method according to claim 1, wherein, depending uponfalling short of the clutch temperature, the emergency operation isdeactivated and normal operation restored.
 3. The method according toclaim 2, wherein a damage index is calculated besides the clutchtemperature.
 4. The method according to claim 3, wherein the damageindex is fed to a routine for monitoring thermal shock of a frictionclutch pressure plate under frictional engagement.
 5. The methodaccording to claim 4, wherein, once thermal shock is detected, theaffected friction clutch is opened for a preset duration of time.
 6. Themethod according to claim 1, wherein in the case of an application withat least one self-opening friction clutch, a self-opening function ismonitored within a preset time interval and upon detecting the absenceof said self-opening function, the emergency operation is initiated. 7.The method according to claim 6, wherein after a default number ofsuccessful self-openings, the emergency operation is deactivated andnormal operation is initiated.
 8. The method according to claim 7,wherein after a default number of failed self-openings a warning isoutput to the driver.